Plasmas, such as are described herein, are partially ionized gases and consist of ions, electrons and neutral species. Plasma is state of matter produced by the action of DC or AC fields having RF or MW frequencies. Plasmas can be useful for treating the surface of a workpiece.
Low power density, low intensity plasmas, such as dark discharges and corona discharges, have been used at low pressure and at one atmosphere for the surface treatment of various materials. However, due to the relatively low energy density, these discharges alter surface properties of materials relatively slowly, if at all. Also, corona discharges are non-uniform and filamentary in nature. This may lead to localized arcs known as “hot spots” and non-uniform surface modification. Generally, the use of corona discharges for treating the surface of a workpiece is unsatisfactory.
Glow discharge plasmas are produced by free electrons energized by an imposed DC or RF electric field. This field causes electrons to collide with the neutral species. These collisions form a variety of active species which may include, excited atoms and molecules, metastables, individual atoms, free radicals, molecular fragments, monomers, electrons and ions, as well as ultraviolet and visible photons
Glow discharge plasma has been successfully used in low pressure or partial vacuum environments (e.g., below 10 Torr). In many applications, glow discharge provides active species which produce important effects, but generates temperatures high enough or requires treatment times long enough to damage the surface of the workpiece being treated.
The placement of the workpiece relative to the plasma zone is critical. If MW plasma is used and the workpiece is too close, it will be damaged by the heat. If the workpiece is too far away, the active species may recombine before encountering the surface of the workpiece.
Plasma treatments, such as plasma etching, deposition, cleaning or sanitization of a workpiece, can change the surface properties of the workpiece. Plasma etching can remove small amounts of material from the surface, thereby also removing surface contaminants and/or exposing a fresh surface for subsequent processing. Plasma etching can be used for applications such as sterilizing, cleaning, etc. of surfaces of a workpiece. Plasma-cleaned surfaces may provide for adhesion of electroplated layers or paint and stronger adhesive bonding of substances to the surface. Plasma etching may be useful for skin cleaning, hair cleaning, window cleaning, etc. Plasma etched surfaces may have modified moisture uptake properties, useful in applications such as household painting, nail polishes, hair coloring, skin moisturizing, etc.
Plasma deposition can deposit a thin layer or coating on the surface of a workpiece, providing improved surface properties such as hydrophobicity, hydrophilicity, oleophobicity or oleophilicity. Such surface properties may provide stain resistant fabrics, or superabsorbent material on the substrate. Plasma deposited coatings can be used on a variety of workpieces such as shoes, carpets, upholstery, decks, etc.
Glow discharge plasmas are typically generated in low pressure environments. This constraint usually necessitates the treatment of workpieces within a vacuum system. Alternatively, glow discharge plasmas may be generated, for example, using the one atmosphere uniform glow discharge plasma reactors described in U.S. Pat. Nos. 5,387,842 and 5,403,453, both issued to Roth, et al, on Feb. 7, 1995, and Jun. 6, 2000. Most of this work has been performed in a laboratory environment and has been limited to very small scale operations. The workpiece is limited to the size of the chamber. To be able to treat a workpiece of any size, such as a garment, the chamber must be large enough to allow the garment to be disposed within the chamber. Generally, this will require the chamber to be big, bulky and expensive.
When treating workpieces, such as garments, the electrodes used to generate the plasma must be spaced a sufficient distance apart if the garment is to be placed between the electrodes. Even if the electrodes are placed on a common side of the workpiece, the electrodes must be sized to generate a plasma field large enough to react with the entire workpiece. This increased electrode spacing and size constraint creates problems with the generation of uniform plasma.
Furthermore, such a chamber is typically stationary and thus requires the workpiece to be moved and placed within the chamber. This type of system cannot be used with a stationary workpiece.
U.S. Pat. No. 5,671,045 issued to Woskov et al. on Sep. 23, 1997, and U.S. Pat. No. 6,081,329 issued to Cohn et al. on Jun. 27, 2000, disclose atmospheric pressure microwave plasma devices. These devices are used to activate atoms for trace element monitoring and analysis of solid, liquid and gas samples, based on the principles of the atomic emission spectroscopy. The device does not include a process chamber for treating a surface of a work-piece. Furthermore, these references fail to teach deposition/polymerization/etching, or other surface modifications.
Bayer et al. in the journal “Surface and Coatings Technology” 116-119 (1999), pp 874-878 reveals a process for plasma deposition of organosilicon monomers using microwave plasma. The monomer is introduced either directly into the plasma or into the remote zone downstream the plasma. The described technique requires low pressure environment to pump the gaseous reaction products out of the reactor and to sustain the plasma. It does not disclose an apparatus or process to plasma treat a workpiece at atmospheric pressure.
A publication in the journal of “Surface and Coatings Technology” 116-119 (1999), pp 879-885, by Karches et al., describes a fluidized bed reactor for plasma-enhanced chemical vapor deposition on powder. The plasma is generated in a reactor using microwave radiation. The reaction gas is fed from the bottom side of the reactor. This reactor requires a vacuum unit to sustain the plasma and allow the plasma polymer deposition with microwave radiation.
One system that utilizes microwave radiation to generate plasma at atmospheric pressure is manufactured by MKS/Astex, of Wilmington, Ma. It includes a magnetron head with power supply, Model No. AX2115-2, a circulator, Model No. AX3120, a dummy load, Model No. AX3030, a SMARTMATCH unit including a matching head and detector module, Model No. AX3060, and a plasma applicator with waveguides, Model No. AX7200. The waveguide provides a pathway for the microwave radiation. The plasma applicator includes a quartz tube forming a chamber and having an outlet. However, this system is not designed to treat surfaces. It is not designed to advantageously admit a working gas into its chamber for the treatment of workpieces. In addition, this system does not suggest how to get the working gas into the chamber without destroying the working gas by the plasma. Due to cost and availability, it is desirable to have a plasma-generating apparatus that uses a microwave radiation source to generate the plasma to treat a surface of a workpiece. The present invention provides an approximately atmospheric pressure, microwave source plasma generating apparatus and method to treat a surface of a workpiece. The present invention also provides such an apparatus and method, wherein the apparatus is movable about the workpiece during treatment.